The present invention relates to an apparatus for measuring the thickness of a sheet and, more particularly, to an apparatus employing the principle of mutual inductance to measure the thickness of a sheet of nonmagnetic material.
Caliper gauges, or more generally, apparatuses to measure the thickness of a sheet, where typically the sheet is a material such as paper, are well-known in the art (see for example U.S. Pat. No. 2,665,333). However, heretofore electromagnetic caliper gauges have been of the type known as self-inductance (see also for example U.S. Pat. No. 3,528,002). Self inductance gauges, in general, comprise a coil of wire wrapped about a U-shaped member of a magnetically susceptible material on one side of the sheet to be measured. A current is passed through the coil creating a magnetic field. On the other side of the sheet is a bar also of a magnetically susceptible material. Both the bar and the coil are maintained at a constant distance from the sheet, through the use of well-known techniques, such as air bearings. Since the coil and the bar are maintained at a constant distance from the sheet, the separation between the coil and the bar is determined by the thickness of the sheet. As the thickness of the sheet varies, the separation between the coil and the bar would also vary. The measurement of the separation between the coil and the bar is based on the principle of self-inductance.
The coil acts similar to an inductor. A capacitor is placed in series with the coil. As is well-known from basic circuit theory, a capacitor in series with an inductor would resonate at a frequency determined by the factor 1/(.sqroot.LC). The coil, however, does not act similar to an inductor with a constant value for its inductance. As the distance between the coil and the bar changes, so does the inductance of the coil. Thus, the resonating frequency of the capacitor in series with the coil is determined by the inductance of the coil, which is determined by the separation between the coil and the bar. The measurement of the resonating frequency would give a measurement for the separation of the coil and the bar. Therefore, the resonating frequency of the circuit gives a measure of the thickness of the sheet. While self-inductance caliper gauges are adequate for some applications, using resonating frequency as a measurement of thickness, they are inadequate for measurement of sheets having large thickness values.
The use of the amplitude of a magnetic field as a measurement of the thickness of a sheet is disclosed in U.S. Pat No. 3,696,290. That patent, however, teaches the use of a u-shaped permanent magnet and a magneto resistor. The u-shaped magnet suffers from the disadvantage that it is not axially symmetrical and thus it is subject to alignment error. Furthermore, unlike an electromagnet whose amplitude can be varied, the amplitude of a permanent magnet cannot be adjusted for varying thicknesses of different sheets, as the gauge is being used.